Velocity effects of barrel length

How much can one expect the velocity to change per inch of barrel? This is mainly asked in the realm of Magnum Calibers(7mm Rem, or 300Win). For example if a reloading manual states velocities achieved using a 26 inch barrel and your rifle only has a 24 inch barrel, how does one estimate the change in velocity. I don't have a chronograph yet so a ball park figure would be nice to know about any given load.

How much can one expect the velocity to change per inch of barrel? This is mainly asked in the realm of Magnum Calibers(7mm Rem, or 300Win). For example if a reloading manual states velocities achieved using a 26 inch barrel and your rifle only has a 24 inch barrel, how does one estimate the change in velocity. I don't have a chronograph yet so a ball park figure would be nice to know about any given load.

Typically the magnums benefit from extra barrel length because of lots of slower burning
powders that they use.

The cases with less than 50 grains of case capacity can use shorter barrels.

So the magnums will lose about 30 to 40 ft/sec for each inch of barrel loss. so you should
lose about 50 to 60 ft/sec going from a 26" to a 24" barrel on the 300 Win.

These velocities will vary some depending on type of powder and the barrel and chamber
combination.